THE RELATIVE IMPORTANCE OF HISTORICAL EVENTS AND GENE FLOW ON THE POPULATION-STRUCTURE OF A MEDITERRANEAN RAGWORT, SENECIO-GALLICUS (ASTERACEAE)

Comparisons of cytoplasmic and nuclear diversity within and among natu ral plant populations have the potential to distinguish the relative i nfluences of seed and pollen dispersal on contemporary gene flow, or a lternatively, may permit inferences of the colonization history of a s pecies via seed. We examined patterns of cpDNA and allozyme variation in Senecio gallicus, a diploid, annual plant that occurs in both coast al and ruderal inland areas of the Iberian Peninsula and southern Fran ce. The species appears to have a strong propensity for long-distance seed dispersal. Five cpDNA haplotypes were found by RFLP analysis amon g a sample of 111 individuals derived from 11 populations. Differences in haplotype frequencies across populations were most evident with re spect to a dramatic increase in the frequency of a derived haplotype f rom coastal to inland localities. The level of cpDNA differentiation a mong populations within the inland group (theta(o) = 0.07) was signifi cantly less than that seen within the coastal group (theta(o) = 0.41). In contrast, for allozymes, no significant difference in population s tructure was evident between collections from coastal and inland habit ats. At the rangewide geographic scale, there was only a very weak ass ociation between inferred levels of gene flow and geographic distance for cpDNA, and no such association was found for allozymes. It appears that while seed movement in the species might be sufficiently great t o disturb the pattern of isolation by distance for cpDNA, it cannot fu lly account for the nearly randomized spatial structure at polymorphic allozyme loci. It is suggested that isolation of populations in Atlan tic-Mediterranean coastal refugia during previous glacial maxima, and the effects of subsequent colonization events in inland areas, have ha d an important effect on molding the present generic structure of the species.